Variable brightness LCD backlight

ABSTRACT

A display for a computer system, such as an LCD, is configured to consume less power when compared to conventional designs. The display includes a screen and at least one backlight configured to illuminate the screen. An input to the at least one backlight is adjustable to produce a desired level of brightness. The input may be computed based on a generated source image and a defined constraint. An input to the display is computed based on the input to the at least one backlight and the source image. The input to the display modifies the level of brightness provided by the at least one backlight to produce a viewable image.

CROSS-REFERENCE TO RELATED APPLICATION

This application relates to, and claims the priority benefit of, U.S.Provisional Patent Application No. 60/606,392, titled “VariableBrightness LCD Backlight,” filed on Aug. 31, 2004. The subject matter ofthis related application is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One or more embodiments of the present invention generally relate tobacklit displays and, more particularly, to reducing power consumptionof backlit displays.

2. Description of the Related Art

Liquid crystal display (LCD) screens, such as the ones used in notebookcomputers or electronic handheld games, are commonly backlit to makethem easier to read. FIG. 1 illustrates an exemplary backlit LCD 100that includes a core of LCD material 102 between sheets of glass 104 and106. A backlight source 108 produces light to illuminate the LCDmaterial 102. As illustrated by the arrows, light produced by backlightsource 108 is generally diffuse, with components traveling in differentdirections. The light from backlight source 108 typically passes througha polarizer 110 that blocks light that is not aligned with an axis ofpolarization of polarizer 110. The light that is aligned with the axisof polarization is allowed to pass through the polarizer 110 to reachthe LCD material 102.

The LCD material 102 has electro-optic properties that causes thepolarized light that passes through the LCD material 102 to twist. Thistwisting may be controlled by applying a voltage waveform to the LCDmaterial 102 for each pixel in an array of pixels. Typically, anelectronic circuit that controls the array of pixels operates byaccepting a digital control value for each pixel in the array of pixels.The control circuit will apply a voltage waveform to the LCD material102 for a pixel based on the digital control value for the pixel.Generally, the control circuit is configured so that smaller digitalcontrol values result in application of a voltage waveform that causesthe LCD material 102 to twist the light in such a way that more of thelight is blocked by the second polarizer 112, thereby causing the pixelto appear darker. Conversely, larger digital control values result inapplication of a voltage waveform which causes the LCD material 102 totwist the light in such a way that less of the light is blocked by thesecond polarizer 112, thereby causing the pixel to appear brighter.

From a power consumption standpoint, conventional LCD backlighting maybe far from efficient. Typically, the backlight source 108 illuminatesall the pixels in the LCD 100 simultaneously with a relatively constantbrightness across all pixels. As previously described, to dim parts ofthe LCD, a voltage waveform is applied to rows and columns of electrodessupported on the glass substrates 104, 106 that causes the LCD material102 to twist in a way that results in more of the light generated by theback light source 108 to be blocked. Dimming parts of the LCD in thisfashion essentially “wastes” a certain amount of the illuminationprovided by the backlight source 108 since the backlight source 108produces the same level of brightness regardless of how much dimmingoccurs on the screen from the voltage waveform. There are manycircumstances where there is a combination of bright and dark images onthe screen, and the dark images may be sustained for some period oftime. Especially in such situations, the conventional way ofilluminating the pixels in the LCD 100 may result in waste. In fact, thepower consumption of a backlit LCD may account for a large portion ofthe overall power consumption of any computer. The inefficiencies due toLCD backlighting may lead to reduced battery life, which may beparticularly problematic, for example, when playing video games orviewing DVD movies on long airline flights.

Therefore, a need exists in the art for a method and system for reducingthe power consumption of backlit LCD displays.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide a method and apparatus foroptimizing the brightness of a backlight that illuminates an LCD,thereby reducing the power consumed.

A “source image” comprising pixel data is provided by a processor. Basedon the brightness information included in the pixel data, an input tothe backlight may be calculated such that the backlight produces a levelof brightness that is at least as great as, but not substantiallygreater than, the brightness of the brightest pixel in the source image.An input to the LCD, the “LCD input image,” is used to modify the levelof brightness produced by the backlight. The LCD input image may becalculated based on the input to the backlight and the brightnessinformation from the source image. Finally, the brightness of the imageproduced on the LCD screen, the “viewed image,” results from thebrightness at each pixel location on the LCD screen being adjusted fromthe level provided by the backlight to a level controlled by the LCDinput image. Ideally, the combination of the backlight brightness andthe LCD input image should make the brightness of the viewed imagesubstantially similar to the brightness of the source image.

In another embodiment, multiple backlight segments are provided toaccount for the fact that there may be significant variation inbrightness across the image displayed on the LCD screen. Each backlightsegment may be driven to produce a different level of brightness. TheLCD input image is determined by considering all the pixels covered byeach of the backlights. Further, the brightness level produced by eachbacklight segment should be at least as great as the brightness of thebrightest pixel it covers, while taking into account the fact that somepixels may be illuminated by more than one backlight segment.

Embodiments of the invention, in calculating the input(s) to thebacklight(s) and the LCD input image, may also account for any backlightsegment that has a known, non-uniform brightness output profile.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 illustrates an exemplary backlit liquid crystal display.

FIG. 2 illustrates a computer system in accordance with one or moreembodiments of the invention.

FIG. 3 illustrates a backlight array in accordance with one or moreembodiments of the invention.

FIG. 4 illustrates a flow diagram of a method for generating a viewedimage in accordance with one or more embodiments of the invention.

DETAILED DESCRIPTION

FIG. 2 illustrates a simplified block diagram of a computer system 200in accordance with one or more embodiments of the invention. Computersystem 200 may be a desktop computer, server, laptop computer,palm-sized computer, tablet computer, game console, cellular telephone,computer based simulator, or the like. The computer system 200 includesa central processing unit (CPU) 202 communicably linked to a system (ormain) memory 210. The system memory 210 may be one or a combination ofmemory devices, including random access memory (RAM), nonvolatile orbackup memory, such as programmable or flash memories, read-onlymemories, and the like.

The computer system 200 may also include an input/output (I/O) interface220, a graphics processing unit (GPU) 230, and a backlight driver module(250). The I/O interface 220 allows the CPU 202 to receive user inputfrom various input devices, such as a keyboard 222 and a mouse 224, viaa bus 208. Alternatively, computer system 200 may include a singlehardwired component or any combination of programmable components, suchas a CPU 202, GPU 230, a video processor (VPU), application processor(APU), or the like.

The GPU 230 is configured to receive graphical information from the CPU202 via the bus 208 and transform the graphical information into asource image (which comprises pixel data) to be sent to a pixel-baseddisplay device 240. Although sometimes referred to herein as an LCD,persons skilled in the art will recognize that the display device 240may be any type of backlit display device, including, withoutlimitation, a conventional CRT, LCD-based monitor, LCD-based projectoror the like. Further, in alternative embodiments of the invention, thesource image may be produced by other types of dedicated hardware, CPU202, programmable hardware, such as a GPU program or a CPU program, orby means external to the computer system 200.

Conventionally, the backlight source operates at a constant brightnessto illuminate the pixels of the LCD material. However, the intensity ofthe backlight need be no greater than is necessary to produce abrightness level that is as great as the brightness of the brightestpixel of the source image. Therefore, pursuant to this invention, thebrightness of the backlight can be continuously adjusted based on thebrightness bitmap associated with the source image. In addition, togenerate the viewed image (i.e., the image produced on the screen of thedisplay device 240), an LCD input image is computed (in one embodiment,by the GPU 230) and used to modify the brightness level at each pixellocation on the screen produced by the backlight. The LCD input imagecomprises a brightness bitmap that is generated based on the input usedto control the intensity of the backlight and the brightness informationfrom the source image. The LCD input image controls the LCD materialwithin the display device 240 (as described above in conjunction withFIG. 1) to adjust the brightness at each pixel location on the screenfrom the level provided by the backlight to a final level. The resultingbrightness of each pixel in the viewed image is substantially similar tothe brightness assigned to that pixel in the source image.

This process can be extended to account for the fact that the backlightmay not have a uniform brightness profile. If the brightness profile isknown, it may be combined with the brightness information from thesource image in calculating the input to the backlight as well as theLCD input image, as described in further detail below in conjunctionwith FIG. 4.

The backlight driver module 250 may be used to generate a signal todrive a backlight array 252 used for illuminating the display device240. According to embodiments of the present invention, the backlightdriver module 250 may also be used to adjust the brightness of thebacklight array 252 based on the source image, as described in moredetail in the following paragraphs.

Referring next to FIG. 3, an especially useful embodiment of theinvention includes two or more backlight sources, each sourcecorresponding to a segment in the backlight array 300. In fact, thebacklight array 252 of FIG. 2 may include any number of individualbacklights configured to illuminate a portion of the display screenhaving a shape that is rectangular, circular, honeycomb, or the like. Itis thus possible to build the backlight array 300 with multiple regionsthat can be lit at multiple brightnesses. This allows darker areas to bepowered down, using less power, rather than using the LCD materialwithin the display device 240 to block out the light in darker areas.For example, in a typical game display, there is oftentimes significantvariation in brightness across the screen. Since this variation iscoherent, it will be possible to dim entire areas of the backlight,thereby reducing power consumption. As FIG. 3 shows, segment A has abrightness profile 320 and segment B has a brightness profile 330. Thisconfiguration introduces the issues of non-uniform intensity across eachsegment and smooth transitions between backlight segments.

Ideally, the backlight segments overlap smoothly, so that there is nosharp boundary in the viewed image where the light from one segment endsand the other begins. It is also possible to practice this inventionwith uniform intensity and/or non-overlapping backlight segments, butless desirable. In the case where the backlight intensity is not uniform(due to multiple overlapping segments and/or nonuniform intensity acrosseach segment), the LCD input image must account for the variations inbacklight brightness. A method for displaying a viewed image when suchnonuniformity in backlight intensity exists is described with respect toFIG. 4. For purposes of discussion only, the backlight array 252 of FIG.2 is assumed to have the configuration set forth in FIG. 3.

The method begins at step 410 where the source image is generated. Atstep 420, inputs to the one or more backlight sources within backlightarray 300 are computed based on the brightness information from thesource image and the brightness profile of each backlight. Specifically,when two backlight sources, A and B, are used, backlight input (IA) forbacklight source A and backlight input (IB) for backlight source B aredetermined. Inputs IA and IB control the illumination provided bybacklight source A and B, respectively. Usually, backlight inputs IA andIB are computed so that the brightness level produced by the series ofbacklight sources A and B is as great as the brightness of the brightestpixel in the area that each such backlight illuminates. In oneembodiment, inputs IA and IB for backlights A and B are computedaccording to the constraint equation:I(x, y)=<IA*BrightnessA(x, y)+IB*BrightnessB(x, y),where I(x, y) is the brightness bitmap associated with the source imageexpressed as a function of pixel position (on the screen of the displaydevice 240), BrightnessA is the brightness profile of backlight Aexpressed as a function of pixel position, and BrightnessB is thebrightness profile of backlight B as a function of pixel position.

In one embodiment, the values for inputs IA and IB may vary from zero toone. Backlight inputs IA and IB may be computed by CPU 202, GPU 230 orother dedicated hardware or programmable hardware, such as a CPU programor a GPU program. In alternative embodiments where the backlight arrayincludes a single backlight source, the above constraint equation issimplified accordingly. Similarly, in alternative embodiments where thebacklight array includes more than two backlight sources, the aboveconstraint equation includes a term for each backlight source.

As previously described, unlike conventional backlights, backlight array300 does not provide a uniform illumination across the display surface.Because the backlight array is more intricate (it has multiplebacklights, which may have different brightness profiles), the LCD inputimage has to be adjusted accordingly. At step 430, the LCD input imageis computed based on the brightness information from the source image,the input to each backlight source in the backlight array and thebrightness profile of each backlight source. In one embodiment of theinvention, the LCD input image, L (x, y), is computed according to theequation:L(x, y)=I(x, y)/(IA*BrightnessA(x, y)+IB*BrightnessB(x, y)).The LCD input image is configured to be used as an input to the displaydevice 240. Like the backlight inputs, the LCD input image may becomputed by CPU 202, GPU 230 or other dedicated hardware or programmablehardware, such as a CPU program or a GPU program.

At step 440, the backlight inputs, IA and IB, are transmitted to thebacklight driver module 250, and the LCD input image, L (x, y), isforwarded to the display device 240. As previously described herein, thedisplay device 240 combines two inputs, the light produced from thebacklight array 300 and the LCD input image, to produce the viewedimage. Specifically, the LCD input image is configured to attenuate, ateach pixel location on the screen of the display device 240, thebrightness associated with the light produced from the backlight array300. This attenuation produces a viewed image having an associatedbrightness bitmap that is substantially equal to the brightness bitmapassociated with the source image.

One advantage of the disclosed systems and methods is that thebrightness associated with the light produced from the backlight may beadjusted according to the source image generated by the GPU 230. Assuch, the power consumed by the backlights in the backlight array variesaccording to each source image generated by the GPU 230, as opposed toremaining constant for all source images, as is the case withconventional systems. Thus, implementing the systems and methodsdescribed herein may substantially reduce the overall power consumptionof computer system 200.

At some pixel positions (e.g., those on the left side of the regionilluminated by backlight A in FIG. 3 and those of the right side of theregion illuminated by backlight B), the algorithm of FIG. 4 has tocontend with only one backlight brightness profile. However, at otherpixel positions (e.g., those in the region illuminated by bothbacklights A and B), the algorithm has to contend with both backlightbrightness profiles. An interesting point about the mathematics is thatthe algorithm works regardless of the number of overlapping brightnessprofiles since the algorithm contemplates a weighted superposition ofthe equations describing the individual brightness profiles of eachbacklight in the backlight array.

One or more embodiments of the invention described above may beimplemented as a program product for use with a computer system such as,for example, the computer system 200 shown in FIG. 2. The programproduct may include a program which, when executed by the CPU 202,performs functions of one or more embodiments of the invention describedherein. The program product can be contained on a variety ofsignal-bearing media, including, but not limited to, non-writeablestorage media (e.g., read-only memory devices, such as CD-ROM disks),alterable information stored on writable storage media (e.g., floppydisks, CD-R/W disks), or information conveyed to a computer by acommunications medium, such as a computer network, telephone network, orwireless network, including the Internet.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A method for displaying a viewed image, comprising: generating asource image having a first brightness bitmap; modifying a first inputto a first backlight and a second input to a second backlight based onthe first brightness bitmap to illuminate a display surface of a displaydevice, wherein the first backlight and the second backlight are part ofa backlight array within the display device; and generating a secondbrightness bitmap associated with the viewed image based on the firstbrightness bitmap and the illumination produced by the backlight array.2. The method of claim 1, wherein the step of modifying the first inputand the second input comprises computing a first intensity gain for thefirst backlight and a second intensity gain for the second backlightbased on the first brightness bitmap.
 3. The method of claim 2, whereincomputing the first intensity gain and the second intensity gain isfurther based on a first brightness profile of the first backlight and asecond brightness profile of the second backlight.
 4. The method ofclaim 3, wherein the first intensity gain and the second intensity gainare computed using the equation,I(x,y)=<IA*BrightnessA(x,y)+IB*BrightnessB(x,y), wherein I(x,y)represents the first brightness bitmap, IA represents the firstintensity gain, IB represents the second intensity gain,BrightnessA(x,y) represents the first brightness profile, andBrightnessB(x,y) represents the second brightness profile.
 5. The methodof claim 3, further comprising the step of computing a third brightnessbitmap associated with a display device input image or LCD input imagebased on the first brightness bitmap, the first intensity gain and thesecond intensity gain.
 6. The method of claim 5, wherein the thirdbrightness bitmap is computed using the equation,L(x,y)=I(x,y)/[IA*BrightnessA(x,y)+IB*BrightnessB(x,y)].
 7. The methodof claim 5, wherein the second brightness bitmap is generated based onthe third brightness bitmap and the illumination produced by thebacklight array.
 8. A system for displaying a viewed image, comprising:a display device having a backlight; and one or more processorsconfigured to carry out the steps of: generating a source image having afirst brightness bitmap; modifying an input to the backlight toilluminate a display surface of the display device based on the firstbrightness bitmap; and generating a second brightness bitmap based onthe first brightness bitmap and the input to the backlight andtransmitting the second brightness bitmap to the display device.
 9. Thesystem of claim 8, wherein a third brightness bitmap associated with theviewed image is generated based on the second brightness bitmap and theillumination produced by the backlight.
 10. A computer-readable mediumstoring instructions that, when executed by a processor, cause theprocessor to display a viewed image, by performing the steps of:modifying an input to a backlight of a display device based on firstbrightness bitmap associated with a source image, wherein the backlightis used to illuminate a display surface of the display device;generating a second brightness bitmap based on the input to thebacklight and the first brightness bitmap; and transmitting the secondbrightness bitmap to the display device to generate a third brightnessbitmap associated with the viewed image, wherein the third brightnessbitmap is based on the second brightness bitmap and the illuminationproduced by the backlight.
 11. A display system for controllablyilluminating a liquid crystal display (LCD) to display a viewed image,comprising: a backlight; an image display surface on the LCD; and aprocessor adapted to: respond to a source image to control the intensityof the backlight so that the brightness of a pixel in the viewed imageon the image display surface approximates the brightness assigned to thepixel in the source image, and define an input image to the LCD based onthe controlled backlight intensity and the source image; wherein abrightness level produced by the backlight is at least as great as abrightness associated with a brightest pixel in the source image.
 12. Adisplay as claimed in claim 11, wherein the backlight comprises at leasttwo segments, and the processor controls the segments to produce abrightness level that is at least as great as a brightness associatedwith a brightest pixel in the source image.